Industrial applications of light olefin-containing hydrocarbons, and particularly liquified propylene, have become more increasingly specialized. The technology as presently developed utilizes highly efficient catalysts to convert these hydrocarbon feedstocks into final product such as polymers. However, these highly efficient catalysts are very sensitive to contaminants, particularly arsenic contaminants, found in these hydrocarbon feedstocks.
In addition to the well known contaminants such as hydrogen sulfide and mercaptans, the light olefin-containing hydrocarbon feedstocks normally contain a small quantity of arsine. Usually arsine is present to the extent of only several hundred parts per million (ppm) by weight. However, even this small amount is normally beyond the allowable limits of an acceptable product.
The presence of arsine, even at very low concentrations, oftentimes renders olefins valueless for many purposes. For example, high purity olefins are required for the satisfactory production of many polymeric products, especially those useful as plastics, including polymers of ethylene, propylene, and the like. However, arsine is a powerful reducing agent which appears able to reduce the olefin polymerization catalysts and cause their deactivation. As a result, there has been a real need to improve techniques for removing arsine from light olefin-containing hydrocarbons, especially those used for polymer production.
Some of the known methods for removing arsine from light olefin containing hydrocarbon streams include the following.
U.S. Pat. No. 3,782,076 (Carr et al., assigned to GULF R & D) discloses a process for reducing the arsenic content, believed to be present as arsine, from gaseous hydrocarbon streams by contacting said streams with supported lead oxide; however, the presence of sulfur compounds is said to interfere with the removal of arsine, and further the supported lead oxide may not be regenerated when sulfur compounds are present in the feed.
U.S. Pat. No. 3,833,498 (Stahfeld, assigned to GULF R & D) discloses a process for reducing the arsenic content, believed to be present as arsine, from gaseous hydrocarbon streams by contacting said streams with activated carbon derived from a bituminous coal and containing cobalt, nickel, molybdenum and vanadium. However, the feed should be substantially dry and free of sulfur compounds.
The purification of propylene and the like olefin feedstreams is particularly complicated by the small difference between the boiling points of propylene and arsine which hampers arsine removal by fractionation. Consequently, the levels of arsine impurity in propylene stocks are oftentimes intolerably high.
Accordingly, it can be seen that there is a need for a process to reduce the arsine concentration in a light olefin-containing hydrocarbon stream to 50 ppb by weight or lower.